1. Field of the Invention
This invention relates to an orthodontic adhesive especially useful for a packaged, adhesive precoated orthodontic appliance.
2. Description of the Related Art
Orthodontic treatment concerns movement of malpositioned teeth to orthodontically correct positions. During treatment, tiny brackets are often connected to the patient's teeth, and an archwire is then secured in a slot of the brackets. The brackets are urged along the archwire by bends or twists in the archwire or by elastic members or other means in order to shift the associated teeth to desired positions.
Previously, orthodontic brackets were connected to teeth by welding or brazing each bracket to a band which was then placed over a tooth in encircling relation. In more recent years, orthodontic brackets have been directly bonded to the tooth surface, resulting in a more aesthetic appearance. Light curable adhesives have been developed which allow the orthodontist to precisely position the bracket on the tooth and then activate a lamp to cure the adhesive and securely fix the bracket in place.
Recent advances in the field of direct bonded dental articles are described in U.S. Pat. Nos. 5,015,180 and 4,978,007, both of which are assigned to the assignee of the present invention. U.S. Pat. No. 5,015,180 describes in one embodiment an orthodontic bracket and a light curable paste sandwiched between a base of the bracket and a flexible, releasably adhering cover sheet. To bond the bracket to a tooth, the cover sheet is removed from the paste and the bracket base is then applied to the tooth. Such construction represents a time savings for the orthodontist, because the orthodontist need not dispense and apply the adhesive paste to the bracket base before bonding the bracket to the tooth.
U.S. Pat. No. 4,978,007 describes in one embodiment a substrate having a recess, an orthodontic bracket having an adhesive on an exterior surface, and a release coating sandwiched between the adhesive and an interior surface of the recess. Such construction is advantageous in that the adhesive is protected in the recess from light, oxygen, water vapor and contaminants. Also, the bracket may be retained in the recess in an upright manner that facilitates grasping of the sides of the bracket by a placement instrument or other tool in order to pull the bracket and adhesive away from the release coating.
Many orthodontists prefer to use certain adhesives that are less viscous (i.e., more fluid) than other adhesives. However, some adhesives with a relatively low viscosity have been found to occasionally distort in shape or remain on the release coating described in U.S. Pat. No. 4,978,007 as the bracket is lifted from the substrate, such that the orthodontist may need to pause to re-shape the adhesive or apply additional adhesive to the bracket base.
Certain orthodontic adhesives having relatively high initial viscosities may release in a satisfactory manner from a release coating when newly mixed. However, brackets that are packaged with a pre-applied coating of adhesive may not be bonded to a patient's teeth for some time. Many conventional adhesives have relatively low molecular weight components that tend to volatilize over a period of time to such a degree that the adhesive becomes too stiff, in general, for satisfactory use for precoated brackets. On the other hand, an adhesive made with relatively high molecular weight components may have a low volatility but may be too stiff to provide sufficient adhesive strength after polymerization.